Plastic lenses are light-weight and hard to crack, and can be dyed, as compared with inorganic lenses, and accordingly, they have been rapidly spreading in optical elements such as spectacle lenses, camera lenses. Various resins for lenses have been developed and used so far, and typical examples thereof include a polythiourethane resin obtained from a polymerizable composition including a polythiol compound and a polyisocyanate compound (see Patent Documents 1 to 5).
In the case of producing a plastic lens including a polythiourethane resin by cast polymerization, an internal mold release agent which contains a phosphoric ester compound or the like may be added to a polymerizable composition in some cases in order to improve the releasability from a cast.
However, in the case of producing a plastic lens using an internal mold release agent including a phosphoric ester compound or the like, white turbidity may be observed in the resin in some cases. In Patent Document 6, it is reported that the white turbidity of the resin can be lessened by heating a mold release agent including a phosphoric ester compound to 80 to 150° C. However, even with the method described in Patent Document 6, the white turbidity of the resin could not be lessened in some cases.